1. Field of the Invention
The present invention relates to multi-carrier transmission systems using orthogonal carriers with high order QAM constellations, in which a receiver sampling clock is phase locked to a pilot carrier. In particular, the invention relates to receivers and transceivers for use with such systems and a method of estimating a pilot carriers phase in such systems.
2. Discussion of the Background
The demand for provision of multi-media and other bandwidth services over telecommunications networks has created a need to transmit high bit rate traffic over copper pairs. This requirement has led to the development of a number of different transmission schemes, such as, ADSL and VDSL. One of the more likely modulation systems for all these transmission schemes is a line code known as DMT (discrete multi-tone), which bears some resemblance to orthogonal frequency division multiplex, and is a spread spectrum transmission technique.
In discrete multi-tone transmission, the available bandwidth is divided into a plurality of sub-channels each with a small bandwidth, 4 kHz perhaps. Traffic is allocated to the different sub-channels in dependence on noise power and transmission loss in each sub-channel. Each channel carries multi-level pulses capable of representing up to 11 data bits. Poor quality channels carry fewer bits, or may be completely shut down.
Because inter pair interference in copper pair cables is higher where data is transmitted in both directions, i.e. symmetric duplex, a number of transmission schemes have proposed the use of asymmetric schemes in which high data rates are transmitted in one direction only. Such schemes meet many of the demands for high bandwidth services, such as, video-on-demand but, in the long term, symmetric duplex systems will be required.
VDSL technology resembles ADSL to a large degree, although ADSL must cater for much larger dynamic ranges and is considerably more complex as a result. VDSL is lower in cost and lower in power, and premises VDSL units need to implement a physical layer media access control for multiplexing upstream data.
Four line codes have been proposed for VDSL:
CAP; Carrierless AM/PM, a version of suppressed carrier QAM, for passive NT configurations, CAP would use QPSK upstream and a type of TDMA for multiplexing (although CAP does not preclude an FDM approach to upstream multiplexing);
DMT; Discrete Multi-Tone, a multi-carrier system using Discrete Fourier Transforms to create and demodulate individual carriers, for passive NT configurations; DMT would use FDM for upstream multiplexing (although DMT does not preclude a TDMA multiplexing strategy);
DWMT; Discrete Wavelet Multi-Tone, a multi-carrier system using Wavelet Transforms to create and demodulate individual carriers, DWMT also uses FDM for upstream multiplexing, but also allows TDMA; and
SLC; Simple Line Code, a version of four-level baseband signalling that filters the base band and restores it at the receiver, for passive NT configurations; SLC would most likely use TDMA for upstream multiplexing, although FDM is possible.
Early versions of VDSL will use frequency division multiplexing to separate downstream from upstream channels and both of them from POTS and ISDN. Echo cancellation may be required for later generation systems featuring symmetric data rates. A rather substantial distance, in frequency, will be maintained between the lowest data channel and POTS to enable very simple and cost effective POTS splitters. Normal practice would locate the downstream channel above the upstream channel. However, the DAVIC specification reverses this order to enable premises distribution of VDSL signals over coaxial cable systems.
Modern multi-carrier transmission systems, using orthogonal carriers with high order QAM constellations for the transmission of multiple bits per carrier and symbol, place high demands on the synchronisation of the receiver with the transmitter. The maximum permitted deviation from exact synchronisation is usually a small fraction of a sampling interval. A reserved carrier, the pilot carrier, which is given a fixed phase, is usually used as the reference to achieve this high accuracy. The receiver sampling clock oscillator is phase-locked to the pilot carrier. It is, therefore, necessary to estimate the phase of the pilot carrier. Using a bandpass filter to recover the pilot carrier, regardless of the frame structure of the DMT signal, does not eliminate the influence of neighbouring carriers on the pilot carrier.
It is an object of the present invention to provide, in a multi-carrier transmission system, a method of estimating the phase of a pilot carrier which exploits the orthogonality of carriers to avoid the influence of neighbouring carriers on the pilot carrier.
It is an object of the present invention to provide, for use with a multi-carrier transmission system, a receiver, arranged to estimate the phase of a received pilot carrier by exploiting the orthogonality of carriers to avoid the influence of neighbouring carriers on the pilot carrier.
It is an object of the present invention to provide, for use with a multi-carrier transmission system, a transciever, arranged to estimate the phase of a received pilot carrier by exploiting the orthogonality of carriers to avoid the influence of neighbouring carriers on the pilot carrier.
According to a first aspect of the present invention, there is provided a receiver, for use with a multi-carrier transmission system using orthogonal carriers with high order QAM constellations, in which a receiver sampling clock is phase locked to a pilot carrier, characterised in that said receiver includes:
selection means adapted to select a frame from a stream of received time domain data;
a one-frequency DFT means for performing a one-frequency DFT on said selected frame, to produce a complex number, representing said pilot carrier, whose argument is the pilot carrier""s phase; and
and processor means for deriving said argument from said complex number.
Said multi-carrier system may be a DMT system.
Said processor means may be adapted to derive said argument from said complex number by an inverse tangent operation.
Said receiver may include correlator means adapted to derive a frame start signal from said stream of time domain data.
Said selection means may be adapted to use said frame start signal to select a frame from the stream of received time domain data.
Said one-frequency DFT means may be a FFT processor.
Alternatively, said one-frequency DFT means may comprise a complex multiplier having:
as a first input a stream of digitized input samples;
as a second input an output from a complex exponential table means; and
an output connected to a complex accumulator; an output from said complex accumulator being said pilot carrier components.
Said complex exponential table means and said complex accumulator may be controlled by signals derived from frame timing logic means.
Said multi-carrier transmission system may be a multi-tone based VDSL system.
According to a second aspect of the present invention, there is provided a transciever, including a transmitter and a receiver, characterised in that said receiver is a receiver as set forth above.
According to a third aspect of the present invention, there is provided a multi-carrier transmission system using orthogonal carriers with high order QAM constellations, in which a receiver sampling clock is phase locked to a pilot carrier, characterised in that said multi-carrier transmission system includes two transceivers as set forth in the preceding paragraph.
According to a fourth aspect of the present invention, there is provided in a multi-carrier transmission system, having a transmitter and a receiver and in which said transmitter transmits a pilot carrier to said receiver, a method of estimating said pilot carrier""s phase, characterised by performing a signal processing operation on portions of a data stream, in which carriers are orthogonal, in order to minimise the effects caused by adjacent carriers.
According to a fifth aspect of the present invention, there is provided in a multi-carrier transmission system using orthogonal carriers with high order QAM constellations, and in which a pilot carrier is transmitted by a first transmitter to a first receiver, a method of phase locking said first receiver""s sampling clock to said pilot carrier, characterised by:
selecting a frame from a stream of received time domain data;
performing a one-frequency DFT on said selected frame, to produce a complex number, representing said pilot carrier, whose argument is the pilot carrier""s phase; and
deriving said argument from said complex number.
Said argument may be derived from said complex number by an inverse tangent operation.
A frame start signal may be derived from said stream of time domain data by a correlation process.
Said frame start signal may be used to select a frame from the stream of received time domain data.
Said one-frequency DFT may be performed by means of a FFT processor.
Alternatively, said one-frequency DFT may include the steps of:
multiplying a stream of digitized input samples by values derived from a table of complex exponential values; and
processing the results of said multiplication in a complex accumulator; to yield said pilot carrier components.
Said multi-carrier transmission system may be a multi-tone based VDSL system.